Large natural gas engines encounter a broad range of fuels. Most of these engines do not operate on commercial grade pipeline gas. Most operate on field grade natural gas obtained directly from the wellhead, or on landfill or digester gas. These fuels have widely varying compositions and, therefore, significantly different knock tendencies.
Engine knock (i.e. fuel self-ignition) can cause premature engine damage and is thus undesirable. Knock most often occurs when engines are operated at or near rated power using low-grade fuels, although other factors can also lead to knock. If necessary, spark ignition timing can be retarded to provide acceptable knock margins when using low-grade fuels, but retarded ignition timing also derates maximum engine power output. Therefore, accurate determination of knock characteristics are important for optimizing ignition timing/power derating for engines operating on low-grade natural gas.
In the past, there have been other methods of analyzing the knock characteristics of gaseous fuels. One procedure uses a mixture of hydrogen and methane as reference fuel blends in test engines to test knock resistance for gaseous fuels. This prior method is the Methane Number (MN) test. However, prior methods have not been applied to non-commercial grade natural gas fuels. Therefore, the prior methods are inaccurate and unreliable when applied to field grade natural gas. The primary constituent field grade natural gas is methane (e.g. usually greater than 60% by volume). Other constituents include other hydrocarbon combustibles such as ethane, propane, n-butane, iso-butane, n-pentane, iso-pentane, hexane, heptane, ethene, propene; non-hydrocarbon combustibles such as hydrogen, carbon monoxide, hydrogen sulfide; inert gases such as carbon dioxide, nitrogen, helium, water vapor; and oxygen. Each of these constituents can have a substantial effect on knock resistance even in small concentrations. Prior methods do not account for the typical range of concentrations of constituents in field grade natural gas, and even ignore some typical constituents altogether.
The present methods are also largely inaccurate for predicting knock tendencies in engines operating on landfill gas or digester gas. The composition of most landfill and digester gases is a mixture of methane and carbon dioxide, but the relative concentrations of methane and carbon dioxide can vary substantially. Carbon dioxide significantly resists knock due to its ability to absorb heat. Prior models do not accurately account for the effects of widely varying concentrations of carbon dioxide which are prevalent in landfill and digester gases.